The invention relates to calenders and like machines in general, and more particularly to improvements in guards for the nips of rolls in such machines.
German patent application No. 38 38 746 of Hinz et al. (published May 17, 1990) discloses a calender with a guard which can be moved to different positions and is caused to change its position prior to separation of the rolls which define the nip. The guard has a first elongated protector which is closely adjacent the periphery of one of the rolls, and a second elongated protector which is closely adjacent the periphery of the other roll. The purpose of such protectors is to prevent penetration of fingers into the nip when the rolls rotate, e.g., in a machine wherein the rolls serve to impart a particular finish to a running web of paper, foil, textile or other material. It is necessary to ensure that the protectors remain sufficiently close to the respective rolls, not only when the machine employing the rolls is in actual use but also during the intervals of nonuse. This is important in many types of paper calenders because the rolls of such calenders continue to rotate even when they are moved apart to increase the width of the nip. Thus, the danger of injury is present irrespective of the width of the nip between the rolls which are adjacent the two protectors.
The rolls of a paper calender can include so-called filled (soft) rolls and so-called hard rolls (e.g., steel rolls). Irrespective of their exact composition, these rolls must be treated from time to time in order to eliminate damage to their peripheral surfaces. This applies especially for the relatively soft filled rolls. The treatment can involve dressing by a turning, grinding or other suitable tool. Such dressing entails a reduction of the diameter of the treated roll; for example, a fresh roll can have a diameter of 825 mm, and the fresh roll can be subjected to repeated dressing or similar treatments so that its diameter is reduced all the way to 675 mm which is still satisfactory for proper operation of the calender.
The aforementioned guard must be adjusted after each reduction of the diameter of a roll. Hinz et al. propose to employ two feed screws, one at each end of the guard, and each of these feed screws must be manipulated by an operator, i.e., two persons must be in attendance to adjust the position of the guard whenever the diameter of a roll has been changed, either as a result of insertion of a fresh roll or as a result of a reduction of the diameter of a used roll. The adjustment must be the same at each end of the guard, i.e., the width of the clearance between each protector and the peripheral surface of the respective roll must be the same, in order to prevent undue reduction of the clearances (i.e., actual contact between the protectors and the adjacent rolls) or an excessive increase of the width of a clearance which could result in injury to a careless operator. The establishment of actual contact between a protector and the adjacent roll could result in damage to the roll. Thus, it is desirable to ensure that the work which is to be performed by two attendants be carried out in synchronism for the purpose of avoiding injury to operators as well as for the purpose of avoiding damage to a roll. Therefore, the persons in charge of adjusting the position of the guard must proceed slowly, i.e., the interval of idleness of a calender or a like machine subsequent to completion of a dressing operation and reintroduction of the treated roll into the machine is very long and contributes significantly to the down times of the machine.